1. Field of the Invention
The present invention relates to a method of detecting faces in an image. More particularly, the invention relates to a method for finding all rotated and upright frontal faces in arbitrarily complex visual scenes.
2. Discussion of the Related Art
A number of systems have been developed that recognize the identity of faces in an image once they are first located in the image scene. Several of these systems utilize methods to detect faces based on such factors as motion in the image scene, color information, or searching for a pre-computed set of features, such as nostrils or other facial features. For example, U.S. Pat. No. 5,012,522 discloses a facial recognition system which searches first for both eyes, then the nose, and then finally, the mouth of a person's face in an image. The disclosure notes that faces are never present when there is no motion of the image. Thus, such a system would be unusable for recognizing or detecting faces in a still photograph.
U.S. Pat. No. 4,975,969 discloses a security system which uses a person's physical characteristics to determine whether to grant access to them. A part of this system involves facial recognition. In this arrangement, the person to be recognized assumes a certain position facing a camera. The camera takes a shot of the face, resulting in an input image of mainly a singular, upright, up-close, and frontal view of a face This makes the facial recognition relatively straightforward. However, it is necessary to first detect whether a facial representation is, in fact, in an image before such an facial recognition can be performed. Moreover, it is often desired to locate multiple faces, which are either upright or rotated, in an image. Further, the faces in the image may often be relatively small in comparison to the entire input image. Even further, the input image may often be cluttered with non-facial backgrounds or scenery. Provisions for first detecting whether faces are present in an image including such non-facial features are desired in facial detection systems.
U.S. Pat. No. 5,642,431 discloses a network-based system and method for detecting faces in an image. This method involves first gathering representatives of an object to be detected, for example, pictures of frontal, upright faces (a class A training set). Representatives of other objects are also gathered, for example, pictures which do not include a face (a class B training set). A single neural network and cluster based system is trained to discriminate between classes A and B. This system is tested on a large number of images which do not contain faces. Any of these images which are mistakenly indicated as including a face are added to the class B training set. This system requires computation of a distance metric, for example, the Mahalanobis distance from a set of six clusters, to accomplish the facial detection. This arrangement requires complex mathematical computations and is time consuming.